1. Field of the Invention
The present invention is directed to a method for transforming a substrate with edge contacts into a ball grid array, a ball grid array manufactured according to this method, and flexible wiring for the transformation of a substrate with edge contacts into a ball grid array
2. Description of the Prior Art
In the manufacturing of multichip modules (MCM), i.e. systems with one or more unencapsulated integrated circuits on a flexible circuit (conductor pattern) film, the problem arises of contacting this module on an element that is a continuation of the circuit. Specially formed edge terminals are conventionally used that are then soldered. e.g. in the strap soldering method, with a circuit board that continues the circuit. As in the case of encapsulated components for surface mounting, the increasing number of terminals and the drive towards miniaturization leads to an ever-smaller terminal grid. This in turn causes difficulties in mounting, from the application of the soldering paste in a defined manner to the short-circuit-free soldering with the soldering strap.
With respect to the above-named difficulties, ball grid arrays (BGAs) are increasingly being used both for individual chips and for multichip modules, in which BGAs the terminals are distributed in planar fashion and thus in an essentially coarser grid than in the conventional forms on the underside of the housing. Each of these planarly distributed terminals supports one or more humps that can be melted, known as balls, which provide the connection with the conductors (circuit) in a melting process (cf. DE-Z productronic 5, 1994, pages 54, 55).
If the wirings of the multichip modules are manufactured in circuit board technology (known as MCM-L), the ball grid array can be realized easily. Through-contacts through the substrate enable a planar arrangement of the terminals or of the meltable humps on the underside. Likewise, given multilayer ceramics manufactured in sinter technology with a thick layer wiring (called MCM-C), a planar arrangement of the terminals or of the meltable humps is possible.
If, however, the multichip modules are manufactured by deposition of layers on an unstructured substrate, e.g. ceramic (called MCM-D), this substrate will only outwardly have edge contacts for the connection; i.e., assembly as a ball grid array is not possible without taking further steps.
From PCT Application WO 89/10005, a chip packaging is known in which a preformed substrate has a receptacle at its upper side for receiving an integrated circuit, and has four ribs on its lower side that project beyond the outer edge. A flexible wiring, with an opening corresponding to the receptacle, is applied on the upper side of the substrate, and is bent downwardly around the ribs such that the ends of the individual conductors form terminals in the region of the ribs. These terminals, however, are edge contacts located underneath; i.e., a realization as a ball grid array is also not possible in this technique.
It is an object of the present invention to provide a method transforming a substrate with edge contacts into a ball grid array in a simple way.
The invention proceeds from a recognition that, based on TAB (Tape Automated Bonding) technology, a flexible circuit (conductor pattern) film can be manufactured with planarly distributed terminals, with the conductors leading outwardly from these terminals having exposed ends, i.e. the flexible foil or film that serves as a support material is either not present in the end region of the conductors or has been removed. A flexible conductor pattern fashioned in this way can then be connected with the underside of the IC substrate, by bending the edges of the conductor pattern up and around the sides of the substrate and the ends of the conductors are contacted to the edge contacts on the upper side of the substrate. For manufacturing the ball grid array, it is then necessary only to apply meltable humps or balls onto the planarly distributed terminals of the flexible conductor pattern. Thus, the flexible conductor pattern enables processing as a ball grid array, with all the advantages that this construction offers over fine-grid edge contacts, particularly in larger sizes. If needed, the entire construction can be covered, extrusion-coated, encapsulated, or provided with a cap.
The support film can be provided with a slot-shaped opening at the end regions of the conductors, which enables a particularly simple construction of the exposed ends of the conductors. In the realization, proven methods of TAB technology can be used.
An additional slot-shaped opening can be provided in the support film at a region of the film which will cover a lower edge of the substrate. This facilitates the bending of the flexible wiring around the lower edge of the substrate.
The conductor layout of the flexible circuit can be covered with solder resist. By means of the protective action of the solder resist, the application of meltable balls and also the connection of the finished ball grid array with a continuing circuit are facilitated.
An adhesive between the upper side of the flexible circuit and the underside of the substrate enables a simple and reliable whole-surface connection between the upper side of the flexible circuit and the lower side of the substrate.